1. Field of the Invention
This invention relates to the production of fire-retardant hardboard from wood fibers. More particularly, it relates to the use of orthophosphate and urea compositions to render the hardboard fire-retardant.
2. Prior Art
As is discussed in Kirk-Othmer, Encylopedia of Chemical Technology, 2nd edition, Vol. 21, pp. 603-624, there are various types of wallboard. The classifications of organic wallboard are hardboard, particle board, insulating board and plywood. Each of these materials are manufactured by its own distinct processes and each material has its own distinct properties. Wallboard has the highest density and is defined as "a panel manufactured primarily from interfelted lignocellulosic fibers, consolidated under heat and pressure in a hot press to a density of 31 lb./ft..sup.3 or greater." Densities can be as high as 75 lb./ft..sup.3. As is implied from the definition there the simultaneous application of heat and pressure is used. The fibers are bound together by a thermosetting resin. Generally from about 1 to about 2% of the fibers is the amount of resin used. Particle board is manufactured by a pressing under heat, however, larger quantities of resin are used, that is, in the order of from 6 to 10% of the solids. In general, the particle boards are less dense than hardboard and have a much lower modulus of rupture. To prevent moisture absorption the external surfaces are coated with a sizing. Sizing is not used in hardboard manufacture. Insulation board is, as the name implies, used to provide wall and ceiling insulation. It is also manufactured by a technique which uses heat and pressure but varies considerably from the pressures used to manufacture either hardboard or particle board. A slurry containing about 1% of solids comprising solids of wood and various by-products from other industries such as sugar cane residues, the paper industry and the like. In some instances chemical treatment with lime, sodium, hydroxide, and sodium sulfite is used. Much of the water is then drained from the slurry and a pulp is then pressed to form a mat of relatively low density in the order of 15 to 25 lbs./ft..sup.3. The remaining water is then removed by drying and the insulation board is very low density and forms a barrier to the transfer of heat. No resin is used to bind the fibers together. Neither particle board nor hardboard have insulating properties. Insulation board, by its nature, does not have good structural properties. Plywood is not closely related to the other materials since thin layers of wood are melely glued together to form a laminate of the desired thickness.
As can be appreciated, there is a need to provide fire-retardant wallboard. Interest in this area has been increased greatly because the general interest in safety has resulted in more stringent building codes in regard to fire-resistant building materials. Not only does a material have to impart fire-retardancy to the wallboard, it cannot detract from the structural properties nor be more hydroscopic than the untreated wallboard. While many substances will impart fire-retardancy to the wallboards, there is a need for a fire-retardant additive which not only is sufficient to impart fire retardancy but which will enable the other properties of the particular wallboard to remain the same and in the case of where resins are used, the additive must not interfere with the action of the thermosetting resin. Additionally, additives which are suitable for one particular class of wallboard are not at all suitable for other types of wallboard because of the different manufacturing techniques involved, the different material utilized in the process and the differences in the properties between the various types of wallboard.
British Pat. No. 1,242,290 published Aug. 11, 1971 discloses a fire-resistant wood-based board similar to a hardboard which includes a fire-retardant agent in the form of granules having a mean diameter not exceeding 2mm and provided with an inert moisture resistant coating which may contain oleophillic substances to an extent not exceeding 8% by weight of the granules. The inorganic retardant is encapsulated so that the water solubility and the diffusion ability in the board are reduced. Suggested fire retardants in the form of granules are ammonium nitrates in combination with ammonium phosphates, an ammonium nitrate-diammonium hydrogen phosphate-potassium phosphate mixture; diammonium hydrogen phosphate; a diammonium hydrogen phosphate-urea composition and a boric acid-borax composition. The moisture resistant coatings suggested are talc-fatty acid derivatives and oil; gypsum and silicones; silica gel-stearate and naphthenates; silica gelstearates -- fatty amines and oil and silica gel -- stearates and paraffin wax. The patent clearly teaches that the moisture resistant layer is essential to provide a board which is both moisture resistant and fire retardant. Additionally, the fire-retardant composition is provided as a distinct layer of granules betwen layers of the wood fibers.
U.S. Pat. No. 3,398,019 relates to insulating wallboard in the form of insulation tiles made from wood products. In the production of the insulation board the wood products are slurried in water and have a solids content in the slurry of about 1% by weight. The water is drained from the solids which are then pressed to form a sheet having a density of from about 0.16 grams/cc to about 0.4 grams/cc. The sheets are then dried at oven temperatures of 270.degree. to 500.degree. F. When the ammonium phosphates are used as fire retardants, the wood insulation tiles discolor due to the decomposition of the ammonium phosphates. Urea is suggested as a decomposition inhibitor for the ammonium phosphate because the temperature used in drying the panels is above the decomposition temperature of the monammonium phosphate, one of the suggested phosphates. Data is given only for monammonium dihydrogen phosphate-urea compositions to prevent discoloration. The urea and phosphates are disclosed as being capable of being added as solids or as solutions. Urea is disclosed as having a tendency to draw moisture to the product and large amounts are to be avoided. Borax and boric acid as well as the phosphates are disclosed in Kirk-Othmer, supra, as imparting fire resistance to insulation board.
It is believed, therefore, that an additive which can be added during the manufacture of hardboard which not only imparts fire retardancy but also does not interfere with the resin and does not detract from the structural characteristics of the hardboard would be an advancement in the art.